Capture build dependencies in package.json

This allows others to install the dependencies for compiling Marathon's
JavaScript. The build must be executed from the root assets directory,
src/main/resources/assets.

    # Work from the root of the assets
    cd src/main/resources/assets

    # Install dependencies
    npm install

    # Build JS
    ./bin/build

Fixes issue #121
Merge pull request #122 from mesosphere/npm-modules

Capture build dependencies in package.json
Compile JS files before running Scala build
Explicitly run Maven tests for script
Return to root directory after compiling JavaScript
Merge pull request #123 from mesosphere/travis-js-compile

Compile JS files before running Scala build
Explain asset compilation in README
Add a checkbox for toggling selection of all tasks
Delimit optional fields with <hr> and a heading

This makes it more apparent which fields are required to create a new
application.

Addresses issue #112.
Move to Option[] for container, same as AppUpdate.
Concludes merge from master.